What is priority inversion?

Question

I\'ve heard the phrase \'priority inversion\' in reference to development of operating systems.

What exactly is priority inversion?

What is the problem it\'s

Answer 1

[ Assume, Low process = LP, Medium Process = MP, High process = HP ]

LP is executing a critical section. While entering the critical section, LP must have acquired a lock on some object, say OBJ. LP is now inside the critical section.

Meanwhile, HP is created. Because of higher priority, CPU does a context switch, and HP is now executing (not the same critical section, but some other code). At some point during HP's execution, it needs a lock on the same OBJ (may or may not be on the same critical section), but the lock on OBJ is still held by LP, since it was pre-empted while executing the critical section. LP cannot relinquish now because the process is in READY state, not RUNNING. Now HP is moved to BLOCKED / WAITING state.

Now, MP comes in, and executes its own code. MP does not need a lock on OBJ, so it keeps executing normally. HP waits for LP to release lock, and LP waits for MP to finish executing so that LP can come back to RUNNING state (.. and execute and release lock). Only after LP has released lock can HP come back to READY (and then go to RUNNING by pre-empting the low priority tasks.)

So, effectively it means that until MP finishes, LP cannot execute and hence HP cannot execute. So, it seems like HP is waiting for MP, even though they are not directly related through any OBJ locks. -> Priority Inversion.

A solution to Priority Inversion is Priority Inheritance -

increase the priority of a process (A) to the maximum priority of any other process waiting for any resource on which A has a resource lock.